One non-invasive tomographic measuring technology that has been used in the medical field, etc., is an optical coherence tomography (OCT) that uses temporally low coherence light as a probe (refer to Patent Literature 1). The OCT, as it uses light as a measuring probe, has the advantage of being able to measure the refractive index distribution, spectrometric information, polarization information (double-refractive index distribution), etc., of the measuring target.
The basic OCT 43 is based on Michelson's interferometer and its principles are explained using FIG. 8. The light output from a light source 44 is parallelized by a collimator lens 45 and then split into reference light and object light by a beam splitter 46. The object light is condensed onto a measuring target 48 via an objective lens 47 inside the object arm, where the light is scattered and reflected and travels back to the objective lens 47 and beam splitter 46.
On the other hand, the reference light passes through an objective lens 49 inside the reference arm and then is reflected by a reference mirror 50 and travels back to the beam splitter 46 through the objective lens 49. The reference light, now back at the beam splitter 46, enters the focusing lens 51 together with the object light to be condensed onto a photodetector 52 (photodiode, etc.).
For the OCT light source 44, a source of temporally low coherence light (type of light that almost never interferes with another light output from the same light source at a different point in time) is used. With Michelson's interferometer that uses a temporally low coherence light as its light source, interference signals appear only when the distance from the reference arm and that from the object arm are roughly equal.
As a result, measuring the interference signal intensity using the photodetector 52 while changing the differential optical path length (τ) between the reference arm and object arm gives interference signals relative to the differential optical path length (interferogram).
The shape of this interferogram represents the reflectance distribution in the depth direction of the measuring target 48, where the structure of the measuring target 48 in the depth direction can be obtained by one-dimensional axial scan. As described above, the OCT 43 allows for measurement of the structure of the measuring target 48 in the depth direction by means of optical path length scan.
This axial scan may be combined with lateral mechanical scan to obtain two-dimensional tomographic images of the measuring target using the resulting two-dimensional scan. The scanning device with which to perform this lateral scan may be constituted so that the measuring target is moved directly, or it may be constituted so that the objective lens is shifted while the target remains fixed, or it may be constituted so that both the measuring target and objective lens remain fixed while the galvano-mirror positioned near the pupillary surface of the objective lens is angularly rotated, or the like.
Extended forms of the aforementioned basic OCT are the spectral domain OCT (SD-OCT) where a spectrometer is used to obtain spectral signals, and the swept source OCT (SS-OCT) designed to obtain spectral interference signals by scanning the wavelength of the light source. The SD-OCT is classified into the Fourier domain OCT (FD-OCT; refer to Patent Literature 2) and the polarization-sensitive OCT (PS-OCT; refer to Patent Literature 3).
The FD-OCT is characterized in that the wavelength spectra of reflected light from the measuring target are obtained using a spectrometer, after which the obtained spectral intensity distribution is Fourier-transformed to retrieve the signals in real space (OCT signal space), and with this FD-OCT, the tomographic structure of the measuring target can be measured by scanning it in the x-axis direction, without scanning it in the depth direction.
The SS-OCT obtains three-dimensional optical tomographic images by changing the wavelength of the light source using a high-speed wavelength-scanning laser and then rearranging interference signals and thus processing the signals using the light source scan signals synchronously obtained by the spectral signals. Those using a monochrome meter as the means for changing the wavelength of the light source can also be used as the SS-OCT.
Measuring the distribution of retinal blood flows using the Doppler optical coherence tomography (Doppler OCT) has been known. This provides a means for measuring the distribution of retinal blood flows using the aforementioned FD-OCT, etc., and similarly for forming cross-sectional images of retinal blood flows as well as observing the three-dimensional vascular channel structure of the retina using the spectral domain OCT.
The inventors of the present invention had previously focused on the Doppler OCT and studied/developed methods to non-invasively measure in-vivo blood flows, particularly those at the back of the eye. The inventors of the present invention had previously succeeded in examining the blood flows at the back of the eye by implementing the Doppler OCT based on the SD-OCT technology, but in recent years the technological base of OCT has been shifting from the SD-OCT to the next-generation OCT, namely, SS-OCT.
One social issue of late is that many cases of glaucoma and age-related macular degeneration (AMD) are being reported among other ophthalmological diseases, and it has become important to find technology to quantitatively evaluate the data measured on tomographic images of the back of the eye for effective diagnosis and treatment of these diseases.
At the back of the eye, the choroidal layer is a layer having many blood vessels (choroidal vessels) present between the retinal pigment epithelium (RPE) and sclerotic coat. Also, the choroidal layer has a function to supply oxygen and nutrients to the outer layer of the retina. Morphological changes to the choroidal vessels are useful in diagnosing glaucoma, age-related macular degeneration (AMD), and other eye diseases associated with circulatory anomaly. In this sense, images of the choroidal vessels are extremely useful in the field of ophthalmology.
Indocyanine green angiography (ICGA) has been traditionally used as a standard means for imaging the choroidal vessels, and use of near infrared light and near infrared light detector achieves greater permeability than when fluorescence angiography is used.
Incidentally, in diagnosing glaucoma, age-related macular degeneration (AMD), and other diseases mentioned above, it is necessary to separate out and distinguish blood vessels, and for this reason many algorithms for separating out the vessels shown on two-dimensional or three-dimensional images have been proposed (refer to Non-patent Literatures 1 and 2).
Separated images of choroidal vessels not only provide intuitively visual pictures of the choroidal vessels, but they also provide quantitative information of their morphology, which is useful in understanding the pathologies of eye diseases associated with blood flows.
Normally, blood vessels are identified using two different means: the tracking-based means (refer to Non-patent Literature 3) and the window-based means (refer to Non-patent Literatures 4 and 5). The tracking-based means is a time-consuming method involving tracing the areas where blood vessels must be extracted, while the window-based means that detects blood vessel boundaries, etc., using a filter requires further improvement of image accuracy.
Recently, a method for automatically extracting the choroidal vessels using etiological detection and regional growth technology was developed, and 3D images have been obtained and the choroidal vessels have been extracted using this method (refer to Non-patent Literature 6).
In addition, a means for automatically separating out blood vessels using a multi-scale 3D edge filter has been proposed (refer to Non-patent Literature 7).